Electrical power generators utilizing natural energy, such as wind turbine generators or photovoltaic units, have attracted attention because of the global environmental problem. However, since their output is highly influenced by weather conditions, it is difficult to generate electricity in response to demand. In addition, since the system voltage and frequency vary depending on the output fluctuation, the level of adoption of such generators is limited due to restrictions in system operation.
In order to solve these problems, recently, hybrid distributed power supply systems have been developed where electrical power generators utilizing natural energy, such as those described above, and electricity storage devices, such as secondary cells, are combined to compensate for the output fluctuation of the electrical power generators with the electricity storage devices, which allows high-quality electrical power to be supplied.
The electricity storage device used in such a hybrid distributed power supply system is described in, for example, Japanese Unexamined Patent Application, Publication No. 2001-327080 (Patent Document 1).
Patent Document 1 discloses a technology where, in order to suppress the fluctuation in output from a distributed power supply to an electrical power system, a target output value is set according to the amount of storage power of an electricity storage device, and an electrical power regulator is controlled so that the output to the power system reaches the output target value.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2001-327080